The invention relates to an ultrasonic imaging method and apparatus and in particular, to an ultrasound imaging method and apparatus for 3-D gynaecologic inspections.
Linear arrays of transducers in which the transducers are placed side by side along a straight line generate images which are corresponding to a scanning plane perpendicular to the transducer frontal emitting and receiving surfaces and in which the dimension in the longitudinal direction of the transducer array is constant for the entire scanning depth. Thus the images obtained are essentially of a rectangular shape.
Convex linear arrays of transducers produce an image having a trapezoidal shape, the dimension of the imaged area and thus of the image obtained increasing with the depth of penetration of the scanning pulse within the target body.
When an oscillating transducer array is provided, it is important that the axis of oscillation is coincident as far as possible with the longitudinal central axis of the array. This is done in order to reduce as far as possible vibrations to the probe due to the oscillating mass. Also, the requested power for oscillating the array of transducers is reduced when the axis of oscillation is very near or coincident with the central longitudinal axis of the array.
With convex linear arrays, the axis of oscillation can be a secant of the array or can be coincident with a line connecting the two ends of the array or even the axis of oscillation could be chosen as to be tangent to the central point of the convex linear array. Thus there are always parts of the array having a considerable radial distance from the axis of rotation which induces considerable angular momentums. Under this condition the power needed for oscillating the convex or arched array is greater than in the plane rectilinear array and thus the probe is subjected to considerable vibrations. In addition, due to the energy law of conservation, the free orientation of the probe, particularly relative to an axis of angular displacement not parallel to the axis of oscillation of the array of transducers, is hindered so that the manipulation of the probe becomes more difficult or uncomfortable.
The above mechanical effect is far from being a negligible detail since it has to be considered that the oscillation is an alternative motion. So the direction of motion of the array of transducers has to reversed each time. This requires deceleration and acceleration of the mass of the transducer. Furthermore the kinetic energy is very high since the frequency of oscillation is very high, particularly when a very high frame rate is desired. According to the above the motor for driving the array of transducers has to furnish sufficient power for rapidly stopping and accelerating the array to a certain speed; consequently the effects of the mechanical inertia on the entire probe form the point of view of the vibrations and of the reduced handiness of the probe are considerable.
A further drawback in using a so called convex array of transducers, that is, an array of transducers aligned along an arched line such that the emission surfaces of the transducers are placed with the axis perpendicular to the emission surfaces oriented along the radial direction of the curved line and away from the centre of curvature, is that the trapezoidal surface covered by the convex array of transducers is very wide so that often problems arises of acoustic coupling in some areas of the said surface.
In gynaecologic imaging, for example of the fetus, the width of the rectangular imaged zone obtained by a normal linear array of transducers is not sufficient in order to image the entire region, particularly at high penetration depths at which at least part of the fetus is located. A trapezoidal imaged area diverging in the direction of penetration of the emitted ultrasound beams would be more suited for the ultrasound fetal examination so that a convex probe would be the better one to be applied. Nevertheless using such a convex probe would cause the drawbacks indicated above if it is desired to carry out a 3-D (three dimensional) image acquisition.